A boring machine for perforating the ground is generally based on a technique of simply circulating a bit (Oscillating method), a technique of not only circulating a bit or a ball cutter but also pressurizing the same (Reverse Circulation Drilling method: ROC), and so on. According to the oscillating method, in a state in which a standard casing having a diameter of 800 to 3000 mm is clamped by a hydraulic chuck, boring is performed by oscillating a cylinder installed rotatably in a left-right direction. According to the ROC method, the ground is bored using a drive rod having a rotary bit or ball cutter installed at its end portion by rotating the bit or ball cutter. The oscillation method can cope with a soft ground condition, that is, a boring work is properly carried out through soft ground such as soil. However, for a hard-boring operation, it is necessary to demolish rocks under the ground by dropping a large-sized hammer, requiring additional equipment such as a pile driver.
Meanwhile, in the RCD method, which is an advanced method compared to the oscillation method from the viewpoint of boring capacity, a soil layer is first dug using an oscillator or a rotator, both a soft rock layer and a hard rock layer are dug by rotating drill rod a specially designed bit attached to its end portion, and air-suctioning circulating water and cloven rocks through a drill rod pipe, followed by hoisting the rocks to the surface of the ground. The RCD method is essentially employed in large-diameter cast-in-place and top-down method for a foundation work.
Korean Patent Publication No. 10-0372049 discloses a boring machine using a crane. The disclosed boring machine includes a drive rod, a tool housing, a breaker, a bit, a case, and an air pressure excavating means. The drive rod transfers water pressure and air generated from a hydraulic drive unit to a digging position along water pressure and air passages. The tool housing is mounted to an end portion of the drive rod and accommodates various structures. The breaker, which is provided at an upper end of the inside of the tool housing, has a piston to strike by water pressure while elevating. The bit, which is vertically movably attached to a lower end of the tool housing, performs a boring operation such that the breaker strikes the piston. The case is inserted into the tool housing to form a passage ranging from the ground surface to the digging position. The air pressure excavating means is connected to the passages and air holes to allow the tool housing to communicate with the bit such that the air pressure supplied from the outside of the tool housing is discharged through a lower portion of the bit.
In the above-described boring machine, the piston of the breaker is driven by a hydraulic drive unit. Thus, as the depth of a bored hole increases, the configuration becomes complicated, and additional equipments for driving the same become bulky. Particularly, since the bit digs soil using air pressure, it is quite difficult to smoothly excavate the soil as the hole becomes deeper. In addition, when air is used as a pneumatic actuator of a piston breaker, a large amount of air is consumed, resulting in a considerable increase in the operation cost.
In the above-described boring machine, the hammer is installed in each drive rod and a water pressure line and a high pressure line are separately formed to operate the piston of the hammer and to rotate a digging unit. A gas chamber of a back head is provided at an upper end of the piston operated by a water pressure supplied through the drive rod. A nitrogen gas is injected into the gas chamber of the back head. When the piston is lowered by the injected nitrogen gas, an impact applied to a target is increased.
As described above, when the gas chamber of the back head is provided at an upper end of the piston, the gas pressure of the gas chamber should be relatively increased to withstand an earth pressure, which becomes increasingly greater as the depth of the bored hole increases. If the pressure of the back head gas chamber is not high enough, the striking force of the bit is undesirably reduced.